Relative impact of environmental variables on the lake trophic state highlights the complexity of eutrophication controls

Overlooked role of long capping time and environmental factors in the plateau lake for impairing lanthanum-modified-bentonite's immobilization to phosphate

Lanthanum-modified-bentonite(LMB) has been applied for eutrophication management as a phosphate(P)-binding agent in many lakes. However, re-eutrophication took place several years or decades later after the first practice of capping due to dynamic environmental factors in the plateau lake. Here, we investigated the effect of long-term capping and integrated environmental factors in the plateau lake including alkalinity, organic matter, disturbance and photodegradation to the LMB immobilization. Long-term LMB immobilization exhibited C accumulation(82.3%), La depletion(53.5%) and lager size effect in the sediment particle, indicating the breakage of La-O-P bonds and the formation of La-O-C bonds over immobilization time. Additionally, pH(8-10) in the plateau lake could enhance the P desorption and decrease P adsorption through electrostatic repulsion enhancement with the zeta potential reduction(7.2 mV). Further disturbance experiment indicated a significant releasing trend of active P and DGT-labile P from the solid phase, pore water to the overlying water after disturbances due to resuspended releasing, particle size and amorphous Fe, Mn and Al's redistribution. Moreover, 31P NMR and EPR results indicated photodegradation after disturbance converted diester phosphate into orthophosphate with long-term LMB immobilization via the oxidation of ·OH in the sediment of the plateau lake. Therefore, management issues for Xingyun Lake may apply to other plateau lakes with low external P input, intermediate depth and intense disturbance.

Clearing floating submerged vegetation leaves: An effective management to stabilize the clear state in shallow lakes?

Achieving sustainable clear states in eutrophic shallow lakes is challenging due to the lag between nutrient load reductions and ecosystem response, often resulting in regime shifts. Submerged vegetation tends to fall off and float to the surface and block light due to the instability of freshly restored lakes, a key feature in influencing whether lakes deteriorate again. However, the mechanisms linking such transient shading to regime shifts remain unclear. This study conducted in situ experiments that quantified the shading effect of floating submerged vegetation leaves. We introduced the novel parameters, light interception coefficients and function that served as a crucial link between experimental findings and numerical models. Notably, we developed an innovative module specifically designed to assess the impacts of different clearing measures on aquatic ecosystems, which had been seamlessly integrated into the PCLake model. This practical model was applied to Xinglong Lake, recently ecologically restored, to simulate variations in key ecological indicators (total phosphorus (TP), total nitrogen (TN), chlorophyll-a (Chl-a), submerged vegetation biomass (DVeg)) and identify regime shift thresholds under different nutrient loads, initial time and time intervals of leaf clearing. The experimental results showed that light interception coefficients exhibited a subtle pattern, initially increasing slightly with water depth before declining, ranging from 0.573 m2/kg to 0.982 m2/kg for Vallisneria natans. The scenarios simulations demonstrated that prolonging clearing intervals from 0 to 120 days resulted in elevated TP, TN, and Chl-a concentrations, accompanied by a decline in DVeg, even causing the lake to a turbid state. Resuming daily clearing after a period of cessation proved ineffective in restoring the lake ecosystem if a regime shift had occurred. As nutrient loads and interception coefficients increased, the time intervals for triggering regime shifts shortened. We conservatively recommended that leaf clearing intervals should not exceed 10 days and ideally begin by March to ensure sufficient light for submerged vegetation. The study provides valuable insights into the impact of transient shading from floating leaves on regime shifts and offers scientific guidance for maintaining shallow lakes sustainably clear.

What drives the changing characteristics of phytoplankton in urban lakes: Climate, hydrology, or human disturbance?

Phytoplankton in shallow urban lakes are influenced by various environmental factors. However, the long-term coupling effects and impact pathways of these environmental variables on phytoplankton remain unclear. This is an emerging issue due to high urbanization and the resultant complex climate, lake hydrology and morphology, human interference, and water quality parameter changes. This study used Tangxun Lake, the largest urban lake in the Yangtze River Economic Belt, as an example to assess for the first time the individual contributions and coupled effects of four environmental variables and fourteen indicators on chlorophyll-a (Chla) concentrations under two scenarios from 2000 to 2019. Additionally, the influence pathways between the environmental variables and Chla concentration were quantified. The results indicated that the Chla concentration was most affected by lake hydrology and morphology, as were the total nitrogen, total phosphorus, and transparency. Especially after urbanization (2015-2019), the coupling effect of human interference, lake hydrology and morphology, and water quality parameters was strongest (18%). This is mainly due to fluctuations in the lake water level and an increase in the shape index of lake morphology, large amounts of nutrients were input, which reduced lake transparency and indirectly changed the Chla content. In addition, due to the rapid development of Wuhan city, the expansion of construction land has led to an increase in impervious surface area and a decrease in lake area. During periods of intense summer rainfall, a substantial amount of pollutants entered the lakes through surface runoff, resulting in decreased lake transparency, and elevated concentrations of nitrogen and phosphorus, indirectly increasing the Chla content. This study provides a scientific basis for aquatic ecological assessment and pollution control in urban shallow lakes.